A flush is a combination of five cards of the same suit. It is worth mentioning that flush can have different variations like Royal Flush, which includes five cards in a row from Ten to Ace of the same suit, or Straight Flush, which is any five cards in a row of the same suit.
That being said, when people refer to a flush, they are talking about a “simple” flush that does not have all the cards in a row, just ones of the same suit. For example, Ah Jh 9h 6h 2h is an ace-high flush, which is a very strong hand.
If both players have the flush, you need to compare the highest card in the combination to determine the winner. So if you have previously mentioned ace-high flush of Ah Jh 9h 6h 2h, you will win against all flushes that are not ace-high. If both players have an ace-high flush, the second card is compared, and so on, until the winner is determent or the pot is split in half if you both have identical combinations.